1. Technical Field
The present invention relates generally to nitride semiconductor substrates (wafers), and in particular to nitride semiconductor substrates that are especially utilizable in the manufacture of such components as light-emitting elements, electronic devices, semiconductor sensors, and surface acoustic wave devices.
2. Description of the Related Art
Components that in recent years are being manufactured using nitride semiconductors include: light-emitting elements such as light-emitting diodes and laser diodes; electronic devices such as rectifiers, bipolar transistors, field-effect transistors, and HEMTs (high electron mobility transistors); semiconductor sensors such as temperature sensors, pressure sensors, radiation sensors, and visible/ultraviolet light detectors; and SAW (surface acoustic wave) devices.
As represented in the schematic cross-sectional view of FIG. 3, such nitride semiconductors are manufactured using an epitaxial nitride semiconductor layer 2 (which in some cases includes multiple layers) having favorable crystalline properties that is grown epitaxially on the principal surface 1S of a nitride semiconductor substrate 1.
Japanese Unexamined Pat. App. Pub. No. 2003-527296 discloses an AlxGayIn1−(x+y)N (0≦x≦1, 0≦y≦1,x+y≦1) substrate having a flat principal surface, and states that it is preferable for the plane of the principal surface to have an off-axis angle inclined at an angle in a range of 1 to 10° from a crystallographic {0001} plane, {11 20} plane, {10 10} plane, or {10 12} plane.
For example, as illustrated by the schematic cross-sectional view of FIG. 2, when the principal surface 1S of the nitride semiconductor substrate 1 is inclined an angle θfrom a low-Miller-index plane CS (for example, a {0001} plane) of that nitride semiconductor crystal, such a substrate is referred to as a misoriented substrate. A substrate that has an off-axis angle of 0° may be referred to as an exact substrate.
Also, Japanese Unexamined Pat. App. Pub. No. 2004-502298 discloses an AlxGayIn1−(x+y)N (0≦x≦1, 0≦y≦1,x+y≦1) substrate having a flat principal surface, and states that it is preferable for the plane of the principal surface to have an off-axis angle inclined at an angle in a range of 0.1 to 10° from a crystallographic {0001} plane.
In order to manufacture a favorable semiconductor device by epitaxially growing a flat and thin nitride semiconductor layer on a nitride semiconductor substrate, it is preferable to use a flat substrate face having a crystallographic low-Miller-index plane (θ=0°) or a predetermined off-axis angle (θ≠0°) relative to that plane.
However, a problem with such nitride semiconductor substrates is that from region to region locally along the substrate principal surface, characteristics such as optical, electrical, and mechanical properties are inadequate or variations occur.
Also, finding the optimum substrate misorientation angle for epitaxially growing a nitride semiconductor layer onto a nitride semiconductor substrate face to manufacture a favorable semiconductor device is not a simple matter.
The reason for this difficulty is that, as illustrated in the schematic cross-sectional view in FIG. 1, without local variation Δθ in the off-axis angle θ of the principal surface 1S of the nitride semiconductor substrate 1 being set down or controlled, variation Δθ in the off-axis angle will differ from substrate to substrate, or the value of Δθ will be insufficient (<0.05°) to make the determination as to optimum misorientation angle.